Apparatus which is exposed to strong radiation can suffer significant changes in its mechanical properties as a result of irradiation damage to the structural materials of the apparatus. One environment in which this problem is particularly pronounced is the core of a nuclear fission reactor. The core assembly of the reactor includes many parts of metal. Some of these parts are in the form of springs of various kinds. For example, the fuel rods of the assembly are typically held within individual openings of grid spacers against at least one spring member, such as a leaf spring, to permit some longitudinal movement resulting from thermal expansion while nevertheless maintaining the lateral spacing of the tubes. In both boiling water and pressurized water reactors the springs of the core assembly are sometimes made of stainless steel or an alloy known as "inconel". However, where the relatively higher neutron capture of stainless steel or inconel cannot be tolerated, it is the practice to use an alloy known as "zircaloy". Furthermore, for spring members which are subjected to relatively intense irradiation, it is the practice to prefer the use of inconel. It has greater resistance to stress relaxation as a result of the irradiation than is the case for zircaloy, even though zircaloy is less neutron absorbent. There is therefore a need for reducing the irradiation stress relaxation of spring material, especially zircaloy.